A route to definitive characterization of an oxido-reductive mechanism for regulating protein biosynthesis was opened by the finding that the reductive activation of a complex-bound valyl- tRNA synthetase can occur in 50-55% ethylene glycol. In this reagent the activation, which probably occurs normally in microseconds, is slowed so that its completion required more than an hour, and it can thus be studied in the manner of an enzymatic reaction. In the ethylene glycol test the reaction is found to require a heat-stable arsenite-binding protein. The protein has been purified from phosphate-containing heat extracts, after addition of arsenite and precipitation from 50% alcohol, by DEAE-cellulose chromatography, and was obtained as a crystalline product from a frozen suspension at -20 degree C in the presence of 4 mM mercuric chloride. In addition to the protein, three dialyzable, coenzyme-like substances promote the enzyme's activation. These can be separated on columns of Sephadex G10. A prominent component of the coenzyme mixture is an unusual flavin which the most recent experiments suggest binds to the heat-stable protein to form a labile arsenite-sensitive disulfide-reducing protein-coenzyme pair.